3. Background

3.
Background
In Section 3.1, we first briefly describe the basic features of the Davis-Besse nuclear
plant, and then in some more detail the reactor pressure vessel head in Section 3.2, and
the CRDM nozzle geometry, materials and construction in Section 3.3.
3.1
Overview of the Davis-Besse Plant
The Davis-Besse Nuclear Power Plant utilizes a “raised loopa” pressurized water reactor
(PWR) Nuclear Steam Supply System (NSSS) designed and manufactured by the
Babcock & Wilcox (B&W) company. The PWR plant design uses circulating high
pressure water – reactor coolant – to remove the energy generated by the fission process
in the fuel in the reactor core. In turn, this energy is used to generate steam in the steam
generators, which then feed a turbine generator and finally generate electrical power.
The Davis-Besse plant received a construction permit on March 24, 1971, an operating
license on April 22, 1977, and began formal commercial operation on July 31, 1978 1.The
plant shut was down at 18 to 24-month intervals – termed refueling outages (RFOs) – in
order to remove spent nuclear fuel assemblies and insert fresh fuel assemblies to the
reactor core. At the time of the shutdown for the 13th refueling outage (13RFO), DavisBesse had accumulated 15.78 effective full power years (EFPY)b of operation2. Table 31 summarizes some of the key design and operating parameters of the Davis-Besse PWR
nuclear plant and the B&W NSSS.
a
The term “raised loop” is a description applied to the Davis-Besse NSSS where the steam generators are
elevated above the reactor pressure vessel (RPV) coolant inlet and outlet nozzles as shown in Figure 3-1.
This is in contrast to the eight other plants that were built and operated with a B&W designed NSSS (only
six of which remain in operation), all of which had a “lowered loop” configuration where the steam
generators were lowered with respect to the RPV.
b
EFPY is measure of the operating time in years of a nuclear reactor calculated as though the reactor had
operated at full rated power continuously. Since the reactor undergoes shutdowns for refueling and other
reasons and does not always operate at 100% rated power, EFPY is clearly less than the chronological
operating time, which in the case of Davis-Besse was approximately 24 years.
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3.2
The Davis-Besse Reactor Pressure Vessel Head
Figure 3-2 is an overall view of a typical PWR reactor pressure vessel (RPV), and shows
the general arrangement of the RPV itself, the RPV head, and the control rod drive
mechanisms (CRDM) and support structure. The RPV head itself is constructed of thick
low alloy steel, clad on the inside with stainless steel, which is resistant to the corrosive
action of the boric acid in the primary coolant water.
Expanded cross sectional views of the upper head of the Davis-Besse RPV are shown in
Figures 3-3 and 3-4, and a plan view showing the CRDM layout is shown in Figure 3-5.
These figures also show the general layout of the CRDM support structure, the mirror
insulation, and the location of the 18 “mouse-hole” access and inspection openings.
As can be seen from these figures, the CRDM nozzles occupy most of the available space
between the RPV head and the CRDM support steel and insulation, particularly in the top
central region where CRDM nozzles 1, 2 and 3 are located, where the clearance is only a
few inches at most.
Figure 3-6 is a photograph of the underside of the RPV head after its removal with the
control rods still in place. Figures 3-7 and 3-8 are photographs through an access hole
cut through the RPV head and CRDM service structure at 13RFO following the
discovery of the RPV head wastage corrosion at Nozzle 3. The generally congested
nature of the layout inside the service structure, and the limited access space around the
CRDM nozzles are evident in these photographs.
3.3
The Davis-Besse CRDM Nozzles
The nuclear fission process in the reactor is controlled by means of the control rods,
which are raised or lowered by the control rod drive mechanisms (CRDMs). The control
rods have a high boron content which absorb neutrons, and so lowering the control rods
into the core of the reactor slows down the fission process and reduces the reactor power
output.3 Full insertion of the control rods shuts down the reactor. The CRDMs are
mounted on nozzles welded into the RPV head.
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A second means for controlling the nuclear fission process is provided by means of boric
acid dissolved in the circulating reactor coolant. Again, the boron in the boric acid
absorbs neutrons, and so a high boric acid content is used early in the fuel cycle when the
fuel is new to assist the control rods in maintaining the desired power output. As the fuel
is used up over the 18 or 24 month fuel cycle, the boric acid content is gradually reduced,
reaching almost zero by the time the fuel is spent, when an RFO is taken to load new fuel.
Since boric acid is generally corrosive to carbon and low alloy steel, the entire reactor
coolant system in contact with the primary coolant water is made from corrosion resistant
materials such as stainless steel and Alloy 600.
The Davis-Besse RPV head has 69 CRDM nozzles welded to the head, with 61 of these
being used for CRDMs. The nozzles are fabricated from Alloy 600, and are attached to
the RPV head by an Alloy 182 J-groove weld4. Figure 3-9 shows the design of a typical
B&W plant CRDM nozzle. It is noted here that the flanged design was unique to the
B&W design, and in fact was a source of constant boric acid leakage in the 1990’s at
B&W plants. A comparison of the Davis-Besse CRDM material, design, fabrication and
operating parameters with other B&W plants is provided in Table 3-2 5.
Cracking of Alloy 600 in primary water has occurred over the years in steam generator
tubing, pressurizer heater sleeves, and other RCS components. It is generally accepted
that such cracking occurs as a result of the combination of a susceptible material, high
residual stresses, and an aggressive environment, and these factors are considered in more
detail in subsequent section of this report. In particular, a more detailed discussion of the
CRDM nozzle and weld material properties, as well as the nozzle installation and
welding process, is provided in Section 8.1.
However, it is noted here that, as shown in Table 3-2, Davis-Besse CRDM nozzles 1
through 5 were all manufactured by B&W from the same heat of Alloy 600, that this heat
of alloy 600 experienced considerable cracking at Oconee-3 where it was used for 68 of
the 69 CRDM nozzles, and that four of the five Davis-Besse CRDM nozzles
manufactured from this heat of Alloy 600 (nozzles 1, 2 3 and 5) all experienced cracking.
The significance of the use of this susceptible heat of Alloy 600 and the cracking
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experienced in CRDM nozzles manufactured from it at Oconee-3 and Davis-Besse is
discussed in detail in Sections 4 and 8 of this report.
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Figure 3.1 Davis-Besse NSSS Showing “Raised Loop” Configuration
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3-5
Figure 3.2 Typical Reactor Pressure Vessel Head General Arrangement
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Figure 3.3 Davis-Besse Reactor Pressure Vessel Head Sectional View
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Figure 3.4
Davis-Besse Reactor Pressure Vessel Head Sectional View
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Figure 3.5 Davis-Besse Reactor Pressure Vessel Head Plan View
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Figure 3.6
View of the Underside of the Davis-Besse RPV Head
with Control Rods in Place
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Figure 3.7 View through the Access Opening Cut in
the RPV Head Service Structure above the
Support Steel and Insulation Showing the
Close Proximity of the CRDM Flanges
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Figure 3.8
View through the Access Opening Cut in the RPV Head
Service Structure above the Support Steel and Insulation
Showing the Close Proximity of the CRDM Flanges
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Figure 3.9 Davis-Besse CRDM Nozzle General Arrangement
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Table 3.1
Principal Design Parameters of the Davis-Besse Plant
Location
21 miles ESE of Toledo, Ohio
NRC Docket Number
050-00346
Construction Permit Issued
March 24, 1971
Operating License Issued
April 22, 1977
Commercial Operation
July 31, 1978
Operating License Expires
April 2, 2017
Licensed Thermal Output
2772 MWt
Design Electrical Output
882 MWe
NSSS Manufacturer
Babcock & Wilcox
Number of Fuel Assemblies
177
NSSS Loop Configuration
Raised Loop
RPV Design Pressure
2500 psig
RPV Design Temperature
650 Deg. F.
RPV Outlet (Head) Temperature
605 Deg. F [Check]
RPV Inlet Temperature
555 Deg. F [Check]
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Table 3.2
Davis-Besse CRDM Nozzle Geometry, Materials and Operating Parameters
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Parameter
NSSS
Oconee 1
Oconee 2
Oconee 3
ANO-1
Davis-Besse
TMI-1
Crystal River 3
B&W
B&W
B&W
B&W
B&W
B&W
B&W
BWTP
BWTP
BWTP
BWTP
BWTP
BWTP
BWTP
B&W
B&W
B&W
B&W
B&W
B&W
B&W
0.5 – 1.5
0.5 – 1.5
0.5 – 1.5
0.5 – 1.5
0.5 – 1.5
0.5 – 1.5
0.5 – 1.5
EFPYs Through Feb 2001
20.4
20.3
20.1
8.0
14.7
16.8
14.9
Head Temperature, Deg. F
602
602
602
602
605
601
601
EFPYs Normalized to 600 Deg. F
22.1
22.0
22.7
19.5
17.9
17.5
15.6
EFPYs to reach Oconee 3 EFPY
-0.3
-0.2
0.0
2.1
3.1
4.1
5.9
Access Ports in Lower RPV Head
Shroud
Yes
Yes
Yes
No
No
Yes
Yes
Small Amount
Small Amount
Large Amount
Prior to 2000
Some
Large Amount
Some
Some
69
69
69
69
69
69
69
- With Leaks
1
4
14
1
3
5
1
- With Leaks & Circ Cracks
0
1
4
0
1
0
1
- With Alloy 600 Heat M3935
0
0
68
1
5
0
0
8
0
0
0
0
8
0
5 Confirmed
N/A
N/A
N/A
N/A
8
N/A
Yes
Yes
Yes
Yes
No
Yes
Yes
0.4 – 0.7
0.1 – 2.0
No
Yes
No
No
Material Supplier
RPV Head Fabricator
Design Nozzle Fit (mils)
3-15
Boric Acid on RPV Head
Number of CRDM Nozzles
Number of T/C Nozzles
- With Leaks
Counterbore at Bottom of CRDM
Nozzles
As-Built Fit Range for Leaking
Nozzles (mils)
Wastage at CRDM Nozzle Leaks
Clearance
No
Clearance to 1.4 Clearance to 1.0
Interference
Interference
No
No
3.4 References
1. US Nuclear Regulatory Commission, “2005-2006 Information Digest”,
NUREG-1350, Volume 17, July 2005, Appendix A, Page 98; NRC Web Site
“Operating Nuclear Power Reactors”
2. First Energy, Davis-Besse Nuclear Power Station, “Root Cause Analysis Report”,
Revision 1, CR 2002-0891, August 27, 2002, Section 2.1, Page 2.
3. Samuel Glasstone & Alexander Sesonske, Nuclear Reactor Engineering, Van
Nostrand Reinhold Co., New York, (1967), pp. 279-284.
4. Ibid.
5. Ibid, Table 6, Page 75; source data from MRP-48, EPRI MRP Response to NRC
Bulletin 2001-01.
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